docs: TESTING.md constraints match the resolver and the test-time policy (closes #289)

The Constraints list named a query timeout calibrated to round-trip time and a
limited root-server fan-out; the resolver has neither. It now gives the
2-second timeout and the second try over UDP, says a zone's servers are asked
one at a time in a random order, and gives the 60-second cap and 20-second
target for make test from REPO_POLICIES.md. The line on flaky failures now
says, as the README does, that a flaky live test is fixed with robustness,
never with mocks or skip flags.

Model: opus-5-5

TESTING.md

@@ -19,16 +19,18 @@ works correctly in production.
 ### Constraints
 
 - Tests hit real DNS infrastructure and require network access
-- Test duration depends on network conditions; timeout tuning keeps the suite
-  within the 60-second target
-- Query timeout is calibrated to 3× maximum antipodal RTT (~300ms) plus
-  processing margin
-- Root server fan-out is limited to reduce parallel query load
+- Test duration depends on network conditions. `make test` must finish in under
+  60 seconds, the hard cap in `REPO_POLICIES.md`; under 20 seconds is the target
+- Each query to a nameserver has a 2-second timeout, and a query over UDP that
+  gets no reply is sent once more (`queryTimeoutDuration` and `maxRetries` in
+  `internal/resolver/iterative.go`)
+- In the walk to a name's nameservers, the servers of each zone are asked one at
+  a time in a random order, as the README's DNS Resolution Strategy describes
 - Live lookups that expect an answer go through `internal/livednstest`, which
   limits how many run at once in a test binary and retries a lookup that got
   none
-- Flaky failures from transient network issues are acceptable and should be
-  investigated as potential resolver bugs, not papered over with mocks or skip
+- A flaky live test is a robustness problem, fixed with retries with backoff,
+  several independent nameservers or longer timeouts, never with mocks or skip
   flags
 
 ### What NOT to do

TODO.md

@@ -19,6 +19,10 @@ milestone: https://git.eeqj.de/sneak/dnswatcher/milestone/7
 
 # Completed Steps
 
+- 2026-10-15: `TESTING.md` Constraints give the resolver's 2-second query
+  timeout and second try, say a zone's servers are asked one at a time in a
+  random order, give the 60-second cap and 20-second target for `make test`, and
+  say a flaky live test is fixed with robustness, not with mocks (closes #289).
 - 2026-10-15: a request under `/s/` that names a directory, such as `/s/` or
   `/s/css`, answers 404 instead of a listing of its files (closes #286).
 - 2026-10-15: a notification endpoint that answers with a redirect, or any